Black smoke rises from a building hit by Syrian government forces' shelling in Aleppo, Syria, earlier this year. / Aleppo Media Center, AMC/AP

by Tom Vanden Brook, USA TODAY

by Tom Vanden Brook, USA TODAY

TEL AVIV, Israel -- The Syrian government has used chemical weapons on rebels, Israel's top military intelligence official said at a security conference on Tuesday.

Brig. Gen. Itai Brun, the head of research and analysis in Israeli military intelligence, said that Syrian President Bashar Assad's regime has used chemical weapons multiple times.

"Shrunken pupils, foaming at the mouth and other signs indicate, in our view, that lethal chemical weapons were used," he said.

He said sarin, a lethal nerve agent, was probably used. He also said the Syrian regime was using less lethal chemical weapons, and that Russia has continued to arm the Syrian military with weapons such as advanced SA-17 air defense missiles.

White House spokesman Jay Carney said the Obama administration has "not come to that conclusion," that the Syrian regime has used chemical weapons. He reiterated Obama's stance that use or transfer of chemical weapons remains "unacceptable."

The White House supports a United Nations effort to investigate previous allegations that Syria has deployed chemical weapons. Carney charged on Tuesday that Assad is blocking such an investigation, while treading carefully on the latest accusation, which Carney said still needs to be investigated and verified.

"What I won't do is jump to the next step and say 'If claims are verified, what action will we take,'" Carney added. "That's speculating and I won't be doing that."

A senior Defense official told USA TODAY the U.S. government is still examining evidence to determine whether chemical weapons have been used. The official spoke on condition of anonymity because the U.S. position has not been announced publicly.

Brun's assessment echoes similar findings from British and French officials, who told U.N. Secretary-General Ban Ki Moon last month that they have evidence Syria used chemical weapons that injured and killed Syrians. Their evidence included soil samples and interviews with witnesses and members of the Syrian opposition.

France and Great Britain asked the U.N. chief to investigate allegations of chemical weapons use in two locations near Damascus on March 19, as well as in the city of Homs on Dec. 23. Ban has appointed a team of chemical weapons experts to investigate the allegations, but the Syrian government has largely blocked the team from doing its work. Syria, meanwhile, has accused rebels of using chemical weapons.

The allegations by Britain, France and Israel that chemical weapons have been used by the Assad regime are not definitive, according to a senior Pentagon official, who was speaking anonymously because the official was not authorized to speak publicly on intelligence issues. Low-confidence assessments by foreign governments of chemical warfare in Syria cannot be the basis for a U.S. intervention, the official said. U.S. intelligence analysts are working with those allies to determine if chemical weapons have been used.

Meanwhile, George Little, Pentagon press secretary, said the United States believes the Assad regime maintains control of its chemical weapons, meaning they have not been passed on to terrorist groups, one of the so-called red lines triggering deeper U.S. involvent in Syria.

U.S. mililtary intervention in Syria to corral chemical weapons would be risky, said Christopher Chivvis, a military analyst the the RAND Corp. It could provoke Assad to use the weapons more widely, or transfer them to terrorist groups, Chivvis said.

More likely in the short term is a lengthy U.N. investigation of the allegations, he said. And there may be pressure mounting from allies for a more robust U.S. response in Syria, which could include training and arming rebel forces, Chivvis said. Currently, aid has been limited to non-lethal gear like radios and night-vision goggles.

"It may become very difficult not to move forward with more aggressive steps," Chivvis said.

President Obama said in Israel last month that Syrian use of chemical weapons would be a "game changer," but he did not specify U.S. action if the use of chemical weapons was confirmed. Defense Secretary Chuck Hagel said here Monday that chemical weapon use would cross a "red line" and that the Pentagon had contingency plans to deal with it.

U.S. officials have not made the determination that chemical weapons have been used by the Syrian government in its civil war. But the Israeli finding is sure to increase calls for U.S. intervention there.

Making an absolute determination of chemical weapon use is difficult, said Gregory Koblentz,a chemical weapons expert at the Council on Foreign Relations. Reports remain "murky," he said, and the British and French have been "forward leaning" in pressing for supporting the rebels so their threshold for proof may be lower than the United States.

Since the United States has declared chemical weapons use a "red line" its standards are higher. "Other countries might have lower standard to justify taking more forceful action," Koblentz said. "We've seen this pattern before."

However, Elizabeth O'Bagy, a Syria specialist at the Institute for the Study of War in Washington, said the United States may be insisting on an unrealistic standard because of its reluctance to intervene in Syria.

"If the British and French are pretty sure this has happened then I find it difficult to believe the United States would not also have this information," O'Bagy said.

Little said Tuesday following Brun's comments that "the United States continues to assess reports of chemical weapons use in Syria. The use of such weapons would be entirely unacceptable. We reiterate in the strongest possible terms the obligations of the Syrian regime to safeguard its chemical weapons stockpiles, and not to use or transfer such weapons to terrorist groups like Hezbollah."

Hagel discussed Syria's alleged possession and use of chemical weapons Tuesday afternoon in Amman, Jordan, during a meeting with Lt. Gen. Prince Feisal of Jordan and Gen. Mashal al-Zaben, the chairman of the Jordanian Chiefs of Staff.

The United States and Jordan agreed to "consult closely on a number of issues," Little said, "including chemical weapons and the demands posed by the influx of Syrian refugees flying the violence."

So far this year, the United States has provided more than $70 million to Jordan to help secure its borders and prevent any chemical weapons transfer, Little said.

Hagel also met in Saudi Arabia with Crown Prince and Defense Minister Salman bin Abdulaziz al Saud Tuesday night. They discussed the sale of "stand-off" weapons, Little said. Those weapons would allow Saudi warplanes to launch attacks from a safe distance, presumably at targets in Iran.